The present invention relates in general to the control of spacecraft and, more particularly to apparatus for use in controlling the orientation of spin-stabilized spacecraft.
It is often necessary to determine and/or modify the attitude of a spacecraft while it is in orbit about a planetary body. For example, it may be necessary to precisely determine the spacecraft's orientation in order that it may be accelerated to a desired velocity or placed in a desired orbit by the application of thrust in the proper direction. A particular equipment such as a communications antenna or a solar cell may require a specific orientation for most efficient operation.
One technique which has been commonly employed for adjusting the orientation of a spacecraft is the use of thrusters mounted on the perphery of the vehicle. Typically the spacecraft is spinning about a spin-axis and is non-spinning with respect to the two other axes. In this case, it is possible to use four thrusters to adjust the orientation of the spin-axis in any direction. In order to fire a thruster in the appropriate direction to secure the desired orientation, it is necessary to accurately determine the spacecraft's attitude and spin rate. Knowing the attitude and spin rate it is possible to correct the orientation of the vehicle by applying thrust at the right angular position along the spin axis. This requires exact timing of thruster firing.